The present invention relates in general to screen printing devices and, more particularly, to a method and apparatus for preparing screens used in such devices.
In typical screen printing processes, ink is applied to a substrate, such as a shirt, poster or decals, through screens which have been prepared in a manner to allow ink to pass through only the desired portions of the screen to form the desired graphic on the substrate. In single, multi-color and four-color process screen printing processes, a separate screen is used for each color of ink which is applied to form the graphic on the substrate. The four-color process differs from multi-color processes in that only four ink colors are used to obtain the desired multi-colored pattern on the substrate.
In most screen printing machines, the screens are clamped on hinged arms which allow the screens to be raised and lowered in relation to the substrate. For example, in manually operated screen printing machines for shirts, the arms which hold the screens are arrayed in a spoke-like fashion and both the screens and the substrate are typically free to rotate to bring successive screens into position over the shirt or other substrate. In automatic screen printing machines, only the substrates rotate, typically in a circular configuration. Each screen is successively positioned over the substrate and is lowered onto the substrate. The ink is then applied through the screen and onto the substrate using a squeegee or pressurized plenum.
Conventional methods for preparing the screens used in screen printing processes have been both time consuming and expensive. One such method involves forming the graphic to be printed as an opaque image on a transparent sheet. Thereafter, an unexposed light-sensitive emulsion or stencil coating is applied to the side of the screen that will contact the substrate to be printed. The graphic on the transparent sheet is then placed over the unexposed emulsion on the area of the screen through which ink flow is desired. The screen is then exposed to a light source which cures or hardens the areas on the screen which are not covered by the graphic on the transparent sheet. The open portions or pores of the screen which are covered with the stencil coating but which are not covered by the graphic on the transparent sheet are fixed in place after the screen is exposed to light. After this exposure to light, the transparent sheet bearing the graphic is removed from the screen and the unexposed stencil coating is removed from the screen by washing the screen with water. Therefore, the portions of the screen that were originally covered with the graphic on the transparent sheet will be open and permeable to the printing ink. At this time, the screen printing screen is ready for use in transferring ink onto the substrate to be printed. This is done by mounting the screen on the screen printing machine and moving the screen into registry over the substrate which is placed on a platen. Ink is then forced through the open pores of the screen onto the underlying substrate.
The above method is disadvantageous in forming a screen printing screen because it can be both expensive and time consuming. This method requires that new artwork, in the form of the transparent sheets, must be formed each time a new graphic is to be printed on an object. Further, in order to ensure the proper orientation of the graphic on the object, the transparent sheet must be properly located or registered with respect to both the printing screen and the printing machine. This problem is accentuated when it is necessary to print a multi-colored graphic on the object which requires a number of different screen printing screens, one for each color in the graphic.
To address the deficiencies in the method described above, another method for preparing a screen printing screen has been developed and is described in U.S. Pat. No. 5,156,089. This method eliminates the need for forming a transparent sheet containing the graphic, but itself contains a number of disadvantages. In this second method, an unexposed light-sensitive emulsion layer or stencil coating is applied to the entire printing surface of a screen as in the previous method. The screen is then placed into an apparatus which prints the graphic directly onto the stencil coating with a liquid ink. In this method, therefore, the applied layer of ink replaces the graphic on the transparent sheet. The printing mechanism is controlled with a computer and prints the graphic dictated by the data provided. After the graphic has been printed on the stencil coating, the stencil coating is cured by exposing it to a light source. The printed graphic acts as an exposure mask or shield so that only the stencil coating which is not covered by the graphic is cured. After the stencil coating has been exposed, the screen is washed to remove the layer of liquid ink and unexposed stencil coating from the screen. Although this method eliminates the need for preparing the graphic on the transparent sheet, it also presents a number of disadvantages.
Utilizing the second method described above requires that the screen be cleaned and degreased prior to beginning the process in order to ensure that the stencil coating properly adheres to the screen. This cleaning and degreasing of the screen adds time and thus expense to the overall process of forming the screen printing screen. Further, it is necessary to ensure that the ink used in forming the graphic is compatible with the underlying light-sensitive stencil coating. Because some commercially available stencil coating materials are incompatible with some commercially available inks, the above process limits the materials that can be used for the coating material and the inks that can be used therewith.
Using the above method also requires that the ink coating placed on top of the stencil coating be sufficiently optically dense to prevent the underlying stencil coating from curing or hardening when exposed to light. If the ink coating is not sufficiently optically dense, the stencil coating under the ink can harden, thus creating an unusable screen printing screen. It is often necessary to apply multiple layers of ink to ensure that a sufficient ink barrier is created so that a usable screen printing screen is created. Providing additional layers of ink on top of the stencil coating adds both time and expense to the process of creating a screen printing screen.
A further disadvantage of the above method results because the ink barrier is applied to only one side of the screen mesh. Because the ink is applied to only one side of the stencil coating, only one side of the stencil coating can thereafter be exposed to light. There is a risk that portions of the opposite side of the stencil coating will be underexposed because the stencil coating, which is typically applied by hand, can vary in thickness. These underexposed portions of the stencil coating can result in certain areas of decreased thickness which can break down during use and allow passage of ink to unintended areas of the substrate. In addition, a partially cured stencil is more difficult to reclaim. In the screen printing business, it is often advantageous to reclaim the screen material when it is no longer necessary to use the stencil that has been applied to the screen. The reclamation process involves using solvents to remove the stencil from the screen. These solvents can actually cause a partially cured stencil to further harden and thus increase the difficulty involved in reclaiming the screen material.
The above described process, and screen printing processes in general, use a polyester fabric for the screen material. This polyester fabric allows light to reflect internally within the fibers. As the light reflects within the fibers and travels along the fibers, it can actually cure portions of the stencil coating lying underneath the applied ink layer. This can result in slightly distorted edges of the stencil as the light cures portions of the stencil that are not intended to be cured. In an effort to reduce the amount of distortion which occurs, different colored meshes or fibers, such as orange or yellow, are used. The use of these different color meshes does reduce the amount of light reflection within the fibers, but also increases the costs of the screen material and thus the cost of the overall process.
Finally, in the above process the stencil coating layer is first applied to the entire screen, which is followed by applying an overlying ink barrier layer to portions of the screen. After curing the stencil coating, the ink barrier layer and the underlying uncured stencil coating are removed by washing the screen with water. However, in washing away the ink barrier and the underlying stencil coating, it has been found that a portion of the cured stencil is washed out as well. This is most often found in the pores of the screen mesh that are only partially filled with the cured stencil coating. As these portions are washed out, the resulting stencil takes on a stepped or saw-toothed appearance. This problem is accentuated when a mesh of a lower density is being used.
Thus, a method and apparatus for preparing a screen printing screen are needed which can overcome the above disadvantages. Specifically, a method for preparing a screen printing screen is needed that will lessen the time needed for preparing such a screen, while at the same time improving the quality of the screen that is made.